diff --git a/Documentation/git-merge.txt b/Documentation/git-merge.txt
index 07f7295ec..689aa4c57 100644
--- a/Documentation/git-merge.txt
+++ b/Documentation/git-merge.txt
@@ -98,6 +98,19 @@ commit or stash your changes before running 'git merge'.
 'git merge --abort' is equivalent to 'git reset --merge' when
 `MERGE_HEAD` is present.
 
+--allow-unrelated-histories::
+	By default, `git merge` command refuses to merge histories
+	that do not share a common ancestor.  This option can be
+	used to override this safety when merging histories of two
+	projects that started their lives independently.  As that is
+	a very rare occasion, no configuration variable to enable
+	this by default exists and will not be added, and the list
+	of options at the top of this documentation does not mention
+	this option.  Also `git pull` does not pass this option down
+	to `git merge` (instead, you `git fetch` first, examine what
+	you will be merging and then `git merge` locally with this
+	option).
+
 <commit>...::
 	Commits, usually other branch heads, to merge into our branch.
 	Specifying more than one commit will create a merge with
diff --git a/builtin/merge.c b/builtin/merge.c
index 30b7bd54f..41467e427 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -64,6 +64,7 @@ static int option_renormalize;
 static int verbosity;
 static int allow_rerere_auto;
 static int abort_current_merge;
+static int allow_unrelated_histories;
 static int show_progress = -1;
 static int default_to_upstream = 1;
 static const char *sign_commit;
@@ -221,6 +222,8 @@ static struct option builtin_merge_options[] = {
 	OPT__VERBOSITY(&verbosity),
 	OPT_BOOL(0, "abort", &abort_current_merge,
 		N_("abort the current in-progress merge")),
+	OPT_BOOL(0, "allow-unrelated-histories", &allow_unrelated_histories,
+		 N_("allow merging unrelated histories")),
 	OPT_SET_INT(0, "progress", &show_progress, N_("force progress reporting"), 1),
 	{ OPTION_STRING, 'S', "gpg-sign", &sign_commit, N_("key-id"),
 	  N_("GPG sign commit"), PARSE_OPT_OPTARG, NULL, (intptr_t) "" },
@@ -1398,9 +1401,12 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	update_ref("updating ORIG_HEAD", "ORIG_HEAD", head_commit->object.oid.hash,
 		   NULL, 0, UPDATE_REFS_DIE_ON_ERR);
 
-	if (remoteheads && !common)
-		; /* No common ancestors found. We need a real merge. */
-	else if (!remoteheads ||
+	if (remoteheads && !common) {
+		/* No common ancestors found. */
+		if (!allow_unrelated_histories)
+			die(_("refusing to merge unrelated histories"));
+		/* otherwise, we need a real merge. */
+	} else if (!remoteheads ||
 		 (!remoteheads->next && !common->next &&
 		  common->item == remoteheads->item)) {
 		/*
diff --git a/t/t3033-merge-toplevel.sh b/t/t3033-merge-toplevel.sh
index 46aadc410..c1379b00c 100755
--- a/t/t3033-merge-toplevel.sh
+++ b/t/t3033-merge-toplevel.sh
@@ -19,6 +19,8 @@ test_expect_success setup '
 	test_commit three &&
 	git checkout right &&
 	test_commit four &&
+	git checkout --orphan five &&
+	test_commit five &&
 	git checkout master
 '
 
@@ -133,4 +135,18 @@ test_expect_success 'merge FETCH_HEAD octopus non-fast-forward' '
 	test_cmp expect actual
 '
 
+# two-project merge
+test_expect_success 'refuse two-project merge by default' '
+	t3033_reset &&
+	git reset --hard four &&
+	test_must_fail git merge five
+'
+
+test_expect_success 'two-project merge with --allow-unrelated-histories' '
+	t3033_reset &&
+	git reset --hard four &&
+	git merge --allow-unrelated-histories five &&
+	git diff --exit-code five
+'
+
 test_done
diff --git a/t/t3412-rebase-root.sh b/t/t3412-rebase-root.sh
index 0b5210572..73a39f292 100755
--- a/t/t3412-rebase-root.sh
+++ b/t/t3412-rebase-root.sh
@@ -133,7 +133,7 @@ test_expect_success 'set up second root and merge' '
 	rm A B C &&
 	test_commit 6 D &&
 	git checkout other &&
-	git merge third
+	git merge --allow-unrelated-histories third
 '
 
 cat > expect-third <<'EOF'
diff --git a/t/t5500-fetch-pack.sh b/t/t5500-fetch-pack.sh
index 9b9bec468..91a69fc33 100755
--- a/t/t5500-fetch-pack.sh
+++ b/t/t5500-fetch-pack.sh
@@ -259,7 +259,8 @@ test_expect_success 'clone shallow object count' '
 test_expect_success 'pull in shallow repo with missing merge base' '
 	(
 		cd shallow &&
-		test_must_fail git pull --depth 4 .. A
+		git fetch --depth 4 .. A
+		test_must_fail git merge --allow-unrelated-histories FETCH_HEAD
 	)
 '
 
@@ -279,9 +280,10 @@ test_expect_success 'clone shallow depth count' '
 test_expect_success 'clone shallow object count' '
 	(
 		cd shallow &&
+		git prune &&
 		git count-objects -v
 	) > count.shallow &&
-	grep "^count: 55" count.shallow
+	grep "^count: 54" count.shallow
 '
 
 test_expect_success 'fetch --no-shallow on full repo' '
diff --git a/t/t6009-rev-list-parent.sh b/t/t6009-rev-list-parent.sh
index 66cda17ef..20e3e2554 100755
--- a/t/t6009-rev-list-parent.sh
+++ b/t/t6009-rev-list-parent.sh
@@ -47,7 +47,9 @@ test_expect_success 'setup roots, merges and octopuses' '
 	git checkout -b yetanotherbranch four &&
 	test_commit eight &&
 	git checkout master &&
-	test_merge normalmerge newroot &&
+	test_tick &&
+	git merge --allow-unrelated-histories -m normalmerge newroot &&
+	git tag normalmerge &&
 	test_tick &&
 	git merge -m tripus sidebranch anotherbranch &&
 	git tag tripus &&
diff --git a/t/t6010-merge-base.sh b/t/t6010-merge-base.sh
index 39b3238da..e0c5f44ca 100755
--- a/t/t6010-merge-base.sh
+++ b/t/t6010-merge-base.sh
@@ -215,11 +215,13 @@ test_expect_success 'criss-cross merge-base for octopus-step' '
 	git reset --hard E &&
 	test_commit CC2 &&
 	test_tick &&
-	git merge -s ours CC1 &&
+	# E is a root commit unrelated to MMR root on which CC1 is based
+	git merge -s ours --allow-unrelated-histories CC1 &&
 	test_commit CC-o &&
 	test_commit CCB &&
 	git reset --hard CC1 &&
-	git merge -s ours CC2 &&
+	# E is a root commit unrelated to MMR root on which CC1 is based
+	git merge -s ours --allow-unrelated-histories CC2 &&
 	test_commit CCA &&
 
 	git rev-parse CC1 CC2 >expected &&
diff --git a/t/t6012-rev-list-simplify.sh b/t/t6012-rev-list-simplify.sh
index b89cd6b07..2a0fbb87b 100755
--- a/t/t6012-rev-list-simplify.sh
+++ b/t/t6012-rev-list-simplify.sh
@@ -71,7 +71,7 @@ test_expect_success setup '
 	note J &&
 
 	git checkout master &&
-	test_tick && git merge -m "Coolest" unrelated &&
+	test_tick && git merge --allow-unrelated-histories -m "Coolest" unrelated &&
 	note K &&
 
 	echo "Immaterial" >elif &&
diff --git a/t/t6026-merge-attr.sh b/t/t6026-merge-attr.sh
index 04c0509c4..ef0cbceaf 100755
--- a/t/t6026-merge-attr.sh
+++ b/t/t6026-merge-attr.sh
@@ -176,7 +176,8 @@ test_expect_success 'up-to-date merge without common ancestor' '
 	test_tick &&
 	(
 		cd repo1 &&
-		git pull ../repo2 master
+		git fetch ../repo2 master &&
+		git merge --allow-unrelated-histories FETCH_HEAD
 	)
 '
 
diff --git a/t/t6029-merge-subtree.sh b/t/t6029-merge-subtree.sh
index 73fc240e8..3e692454a 100755
--- a/t/t6029-merge-subtree.sh
+++ b/t/t6029-merge-subtree.sh
@@ -49,7 +49,7 @@ test_expect_success 'setup' '
 
 test_expect_success 'initial merge' '
 	git remote add -f gui ../git-gui &&
-	git merge -s ours --no-commit gui/master &&
+	git merge -s ours --no-commit --allow-unrelated-histories gui/master &&
 	git read-tree --prefix=git-gui/ -u gui/master &&
 	git commit -m "Merge git-gui as our subdirectory" &&
 	git checkout -b work &&
diff --git a/t/t6101-rev-parse-parents.sh b/t/t6101-rev-parse-parents.sh
index 10b145276..1c6952d04 100755
--- a/t/t6101-rev-parse-parents.sh
+++ b/t/t6101-rev-parse-parents.sh
@@ -19,7 +19,7 @@ test_expect_success 'setup' '
 	git checkout --orphan tmp &&
 	test_commit start2 &&
 	git checkout master &&
-	git merge -m next start2 &&
+	git merge -m next --allow-unrelated-histories start2 &&
 	test_commit final &&
 
 	test_seq 40 |
diff --git a/t/t9400-git-cvsserver-server.sh b/t/t9400-git-cvsserver-server.sh
index d708cbf03..432c61d24 100755
--- a/t/t9400-git-cvsserver-server.sh
+++ b/t/t9400-git-cvsserver-server.sh
@@ -45,7 +45,8 @@ test_expect_success 'setup' '
   touch secondrootfile &&
   git add secondrootfile &&
   git commit -m "second root") &&
-  git pull secondroot master &&
+  git fetch secondroot master &&
+  git merge --allow-unrelated-histories FETCH_HEAD &&
   git clone -q --bare "$WORKDIR/.git" "$SERVERDIR" >/dev/null 2>&1 &&
   GIT_DIR="$SERVERDIR" git config --bool gitcvs.enabled true &&
   GIT_DIR="$SERVERDIR" git config gitcvs.logfile "$SERVERDIR/gitcvs.log" &&